Bolt styles are set by various industry standards.
Here are the main ones:

JIS is Japan Industry Standardxxxx used by Japan, China, Taiwan, Korea, othersDIN is Deutsches Institut für Normungxxxx used by Germany, Austria, Netherlands, othersISO is International Standards Organizationxxxx used worldwideANSI is American National Standards Institutexxxx used by America

Metric threads: The way a screw or bolt is described here (the syntax) is “thickness dash thread width times shank length”. So an M6-1.00 x 20 bolt is just under 6mm thick, each thread is 1.00mm wide, and the length below the head is 20mm. By knowing the millimeters and thread pitches, you can find screws and parts for anything from carburetors to telescopes.

3.0 mm bolts (0.60 threads) and 3.5 mm bolts (0.60 threads)

M3-0.60 metric thread is close to #5-40 American. #5-40 has a 3.17 shank diameter (not 3.0), and thread width 0.635 (not 0.60). #5-40 nuts screw onto M3-0.60 bolts. Because the nut is bigger, the slightly different thread is tolerated, so it falsely feels correct. But under load it would strip easier because not all of the threads are in use. M3-0.60 nuts will not screw onto #5-40 bolts.

4 mm bolts (0.75 coarse threads)

5 mm bolts (0.75 fine threads)

M5-0.75 x 12 slot$8for Dellorto SHA carburetor inlet banjo

5 mm bolts (0.80 threads)

M5-0.80 metric thread is very close to #10-32 American. #10-32 has a 4.8 shank diamteter (not 5.0), and 0.79 thread width (not 0.80). M5 is slightly larger. A M5 nut screws onto a #10-32 bolt easily, and can wiggle. Conversely, a #10-32 nut becomes too tight on a M5 bolt, and can only screw on 1 or 2 turns.